I. Field of the Invention
The present invention is directed to an improvement in vehicle-mounted police radar warning receivers, and more particularly to such receivers which detect police radar signals before the vehicle is within the detection range of the police radar unit.
II. Description of the Prior Art
Police radar generally operates in the X-band and K-band of the frequency spectrum as discussed in U.S. Pat. No. 4,313,216, assigned to Cincinnati Microwave, Inc., the assignee herein. There are, generally, two types of police radar. One emits a continuous radar signal so long as the radar unit is turned on. The other emits a brief burst of radar signal when the police officer triggers the unit. This latter type is referred to as pulsed or instant-on radar. While transmitting, both continuous and pulsed radar transmit a signal which is at a fixed frequency within the selected band.
An electronic assembly referred to as a police radar warning receiver has been devised to detect the presence of police radar signals. The receiver is mountable in a vehicle, such as a passenger car or truck, motorcycle, boat or the like, which travels on land or water in areas subject to speed-monitoring radar surveillance by police, and functions to detect the presence of the police radar and provide the driver or user with an audible and/or visual indication that his speed is being checked by radar. The receiver is contained in a box-like housing which is set on the dash or clipped to the visor in the vehicle. Extending from the rear of the housing may be a power cord which terminates in a plug adapted to be received in the cigarette lighter socket of the vehicle. The front panel of the receiver faces the driver and has various indicators and control knobs.
When police radar is operating within range of the radar warning receiver, the circuitry of the receiver is able to detect the presence thereof. The ESCORT and PASSPORT radar warning receivers, manufactured by the assignee herein, Cincinnati Microwave, Inc. of Cincinnati, OH utilize a superheterodyne circuit for this purpose. As explained in aforementioned U.S. Pat. No. 4,313,216, and in U.S. Pat. Nos. 4,581,769 and 4,750,215, which are also assigned to the assignee herein, a superheterodyne circuit employs two local oscillators, one of which sweeps in frequency over a range of frequencies related to one or both radar bands. To this end, a first local oscillator signal is mixed with the incoming police radar or other signal to produce a first IF signal. The first IF signal is then mixed with the second local oscillator signal to produce a second IF signal. Due to the sweep of one of the local oscillators, the second IF signal presents a sweep pattern which extends over a band corresponding to the X and K bands and including noise and signals, the latter at locations in the sweep pattern corresponding to the frequency of received signals in the X and/or K bands.
The sweep pattern of the second IF signal is then passed through an FM discriminator circuit. The FM discriminator provides a second sweep pattern output including S-curves having positive- and negative-going portions to define time-related positions relative the start of the sweep corresponding to the frequency at which an incoming signal is received. As is understood, such a heterodyning process will result in generation of a "duplicate" or image of the police radar signal within the receiver. Hence, the discriminator generates one S-curve related to the actual frequency signal received as well as a second S-curve related to the image frequency of the received signal. One or both of these S-curves may then be utilized to indicate reception of a police radar signal so as to alert an operator to the presence of police radar surveillance as described in aforementioned U.S. Pat. Nos. 4,581,769 and 4,750,215, and U.S. Pat. No. 4,862,175, also assigned to the assignee hereof.
The ability of radar warning receivers to detect police radar signals, however, is limited primarily by the sensitivity of the receiver electronic circuitry which defines a capture range. That is, signals emitted by police radar units may travel a substantial distance from the unit. As is well understood, the police radar signal must travel to the vehicle under surveillance and then return altered by a Doppler shift representing speed of the vehicle. However, as the police radar signal travels to and from the vehicle under surveillance, the weaker the signal becomes. Thus, the further the vehicle under surveillance is from the police radar unit, the weaker the return radar signal is such that at some distance and beyond, the police radar signal is too weak to return to the police radar unit and be evaluated for speed of the vehicle (detection range).
It is desirable that the radar warning receiver detect the police radar signal while it is still so weak as to be beyond the detection range of the police radar unit. However, as with the police radar unit, the further the police radar warning receiver is from the source of the police radar signal, the weaker the signal. At some distance from the police radar unit corresponding to the capture range of the police radar warning receiver, the police radar signal may be so weak that the police radar warning receiver is unable to distinguish signal from noise, meaning that a police radar signal will not be detected until the vehicle moves closer to the police radar unit. The difference in distance between the detection range of the police radar unit and the capture range of the police radar warning receiver defines a reaction zone during which the vehicle operator must react to the presence of a police radar signal. It is desirable that the reaction zone be as large as possible so that the operator of the vehicle under surveillance will have sufficient time to react before the vehicle comes within the detection range of the police radar unit.
Additionally, some police radar units are of the "instant-on" type meaning that they may be used in a manner to intermittently emit only short bursts of police radar signals. Where the bursts are given only infrequently, the first burst may be given when the police radar warning receiver is too far away to detect that burst, i.e., at that distance, the signal from the police radar unit is below the threshold of the receiver. The second burst may come after the vehicle is within the detection range of the police radar unit. Under such circumstances, the operator will have had no advance warning that the vehicle is under surveillance. Accordingly, it is desirable to extend the capture range of the police radar warning by providing the receiver with as much sensitivity, i.e., as low a threshold, as possible so that police radar signals may be received as far from the police radar unit's detection range as possible.
Typical of current radar warning receivers is that their sensitivity is generally low enough to be able to detect most police radar signals somewhat beyond the detection range of the associated police radar unit and, thus, provide a reaction zone. However, greater improvement is desired. One approach could be simply to select a lower threshold above which all signals are accepted as valid, thus extending the capture range. However, this approach may allow too much random noise to pass through the receiver circuitry and appear as police radar signals resulting in irritating and misleading false alarms. Another approach has been to continuously vary the threshold based on the level of random noise as a sweep of the local oscillator progresses. However, in such an approach, the threshold may be varied part way through the sweep but before a weak police radar signal is about to be received. Thus, if the threshold were caused to increase as the sweep progressed, the vehicle may be too far away to detect a weak police radar signal. As any signal not above the threshold is discarded, a police radar signal may be missed. Thus, rather than increase the range of the receiver, such an approach may actually reduce the capture range capabilities thereof, because the sensitivity of a receiver with such a varying threshold may be effectively, but undesirably, reduced. Alternately, the threshold may be caused to decrease to such a level that noise may lead to undue false alarms.